Fiber laser based comb sources are becoming increasingly the laser of choice for applications in precision spectroscopy. Many such applications demand large comb spacing on the order of a GHz or several GHz, precision comb control via adjustable repetition rates, as well as carrier phase control. Moreover, the timing jitter and the carrier phase noise of such fiber combs should be minimal.
Many applications for comb sources have indeed been identified and can comprise, for example, high precision frequency synthesis, all optical clocks (as described in T. Udem et al., ‘Optical frequency metrology’, Nature, vol 416, pp. 233 (2002) and frequency rulers for spectrograph calibration (as described in C. H. Li et al., ‘A laser frequency comb that enables radial velocity measurements with a precision of 1 cm/s’, Nature, vol., 452, pp. 610 (2008). Other applications have been identified in I. Hartl and M. E. Fermann, Laser based frequency standards and their applications, U.S. Pat. No. 7,809,222.
Another important application involves low phase noise micro-wave or radio-frequency sources for atomic frequency standards, radar and remote sensing as described in A. Bartels et al., Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references, Optics Letters, Vol. 30, Issue 6, pp. 667-669 (2005).
Notwithstanding numerous practical advances in fiber comb technology, fiber comb sources with large comb spacing are still difficult to manufacture and not readily applicable to mass production. Moreover, the generation of broad frequency spectra with widely spaced comb lines is challenging. Development of practical comb sources for use in the mid-IR spectral region remains particularly difficult.